A paper titled "Observation of antiprotons," by Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis, members of what was then known as the Radiation Laboratory of the University of California at Berkeley, appeared in the November 1, 1955 issue of Physical Review Letters. It announced the discovery of a new subatomic particle, identical in every way to the proton  except that its electrical charge was negative instead of positive.

In 1955 the newly commissioned Bevatron was the most powerful particle accelerator in the world, capable of propelling protons to energies of some 6.5 billion electron volts.

The history of science is filled with stories about serendipitous discoveries, but the antiproton is not one of those. Its discovery was the culmination of a hunt whose origins can be traced back to 1928, when the eccentrically brilliant British physicist Paul Dirac formulated a theory to describe the behavior of relativistic electrons in electric and magnetic fields.

Dirac's equation was unique for its time because it took into consideration both Einstein's special theory of relativity and the effects of quantum physics, as proposed by Erwin Schrödinger and Werner Heisenberg, to describe the behavior of slow-moving particles. While the math worked, few physicists gave Dirac's equation much serious consideration, because it allowed particles of negative energy. From the standpoint of both physics and common sense, the energy of a particle could only be positive.

Attitudes changed dramatically in 1932 when a young professor at the California Institute of Technology, Carl David Anderson, working on a project that had originated with his mentor, Robert Millikan, reported the observation of a positively charged electron, which he dubbed the "positron."

Both Dirac (1933) and Anderson (1936) would win Nobel Prizes in physics for their discoveries. The existence of the positron, the antimatter counterpart of the electron, nevertheless begged the question of an antimatter counterpart to the proton.

As Dirac's theory continued to prove itself highly successful for explaining electron and positron phenomena, it followed  from the revised standpoints of both physics and "common sense"  that it should also be successful at explaining protons, and would then demand the existence of an antimatter counterpart. The search for the antiproton was underway, but it got off to a very slow start. It would be another two decades before a machine capable of producing such a particle would become available.

The Bevatron

Anderson discovered the positron by probing cosmic rays with a cloud chamber; it would have been extremely difficult if not impossible to use the same approach for finding an antiproton. If physicists were going to find the antiproton, they were first going to have to make one.

Berkeley physicists Edwin McMillan and Edward Lofgren, shown here on the shielding of the Bevatron, were part of the team assembled by Ernest Lawrence to ensure that the new synchrotron would be able to reach the 6.5 BeV needed to produce an antiproton.

Even with Ernest O. Lawrence's invention of the cyclotron in 1931, earthbound accelerators weren't up to the task. Physicists knew that the creation of an antiproton would necessitate the simultaneous creation of a proton or a neutron. Since the energy required to produce a particle is proportional to its mass, the creation of a proton-antiproton pair would require twice the proton rest energy, or about 2 billion electron volts. Given the fixed-target collision technology of the times, the best approach for making 2 billion electron volts available would be to strike a stationary target of neutrons with a beam of protons accelerated to about 6 billion electron volts of energy.

In 1954, Lawrence commissioned the Bevatron accelerator at his Rad Lab. (Upon Lawrence's death in 1958, the lab was renamed Lawrence Berkeley Laboratory in his honor.) This weak-focusing proton synchrotron was designed to accelerate protons up to energies of 6.5 billion electron volts. At the time, around Berkeley, a billion electron volts was designated BeV; it's now universally known as GeV.

Though this was never its officially stated purpose, the Bevatron was built to go after the antiproton. As Chamberlain noted in his Nobel lecture, Lawrence and his close colleague, Edwin McMillan, who codiscovered the principle behind synchronized acceleration and coined the term "synchrotron," were well aware of the 6 billion electron volts needed to produce antiprotons, and they made certain the Bevatron would be able to get there.

Armed with a machine that had the energetic muscle to make antiprotons, Lawrence and McMillan put together two teams to go after the elusive particle. One team was led by Edward Lofgren, who managed operations of the Bevatron. The other was led by Segrè and Chamberlain.

Segrè had been the first student to earn his degree in physics at the University of Rome under Enrico Fermi. He had, with the aid of one of Lawrence's cyclotrons, discovered technetium, the first artificially-produced chemical element. He was also one of the scientists who determined that a bomb based on plutonium was feasible, and his experiments on the scattering of neutrons and protons and proton polarization broke new ground on understanding nuclear forces. Chamberlain had also studied under Fermi and under Segrè as well. He was Segrè's assistant on the Manhattan Project at Los Alamos while still a graduate student and later joined Segrè at Berkeley to collaborate on nuclear forces studies.

Making an antiproton was only half the task. Posing no less formidable a challenge was devising a means of identifying the beast once it had been spawned. For every antiproton created, 40,000 other particles would also come into existence. The time to cull the antiproton from the surrounding herd would be brief: within about a 10 millionth of a second after it appears, an antiproton comes into contact with a proton and both particles are annihilated.

An antiproton speed trap

From left, Emilio Segrè, Clyde Wiegand, Edward Lofgren, Owen Chamberlain, and Thomas Ypsilantis, members of the team that discovered the antiproton

According to Chamberlain's Nobel lecture, it was understood from the start that at least two independent quantities would have to be measured for the same particle in order to identify it as an antiproton. After considering several possibilities, members of the Segrè and Chamberlain team decided that the two quantities to be measured should be momentum and velocity.

To measure momentum, the team used a system of magnetic quadrupole lenses suggested to them by Oreste Piccioni, an expert on quadrupole magnets and beam extraction who was then at Brookhaven National Laboratory. The idea was to set up the system so that only particles of a certain momentum interval could pass through. As the Bevatron's proton beam struck a copper block target, fragments from collisions with nuclei would emerge in all directions. While most of these fragments were lost, some would pass through the system. For specifically defined values of momentum, the negative particles among these system-captured fragments would be deflected by the magnetic lenses into and through collimator apertures.

This first image of an annihilation star, found in the photographic emulsion stack experiments led by Gerson Goldhaber of the Segrè group, confirmed the discovery of the antiproton. An antiproton enters from the top of the image and travels about 430 micrometers before meeting a proton. Nine charged particles emerge from their mutual annihilation.

To measure velocity, which was used to separate antiprotons from negative pions, the Berkeley researchers deployed a combination of scintillation counters and a pair of Cerenkov detectors. The scintillation counters were used to time the flight of particles between two sheets of scintillators spaced 12 meters apart. Under the specific momentum defined by Segrè, Chamberlain, and their collaborators, relativistic pions traversed this distance 11 nanoseconds faster than the 51 nanoseconds it took for more ponderous antiprotons. Signals from the two scintillators were set up to coincide only if they came from an antiproton. However, because it was possible for two pions to have exactly the right spacing to imitate the signal from an antiproton, the researchers also used the Cerenkov detectors.

One Cerenkov detector was somewhat conventional, in that it used a liquid fluorocarbon as the medium. It was dubbed the "guard counter" because it could measure the velocity of particles moving faster than an antiproton. The second detector, which was designed by Chamberlain and Wiegand, used quartz as the medium, and was set off only by particles moving at the speed predicted for antiprotons.

In conjunction with momentum and velocity experiments, a related experiment using photographic emulsion stacks was conducted under the leadership of physicists Gerson Goldhaber from Berkeley and Edoardo Amaldi from Rome. If a suspect particle was truly an antiproton, the Berkeley researchers expected to see the signature star image of an annihilation event, in which both the antiproton and a proton or neutron from an ordinary nucleus (presumably that of a silver or bromine atom in the photographic emulsion) died simultaneously.

Success

The antiproton experiments of Segrè and Chamberlain and their collaborators began the first week of August in 1955. Their first run on the Bevatron lasted five consecutive days. Lofgren and his collaborators ran their experiments the following two weeks. The Segrè and Chamberlain group returned on August 29 and ran until the Bevatron broke down on September 5.

An antiproton (blue) enters a bubble chamber from bottom left and strikes a proton. The released energy creates four positive pions (red) and four negative pions (green). The yellow streak at the far right is a muon, a decay product of the adjacent pion. (The dark blue curlicues are low-energy electrons knocked from atoms, not involved with the antiproton.)

On September 21st, a week after operating crews had revived the Bevatron, Lofgren's group was to begin a four-day run but instead ceded its time to Segrè and Chamberlain. That day, the future Nobel laureates and their team got their first evidence of the antiproton based on momentum and velocity. Subsequent analysis of the emulsion-stack images revealed the signature annihilation star that confirmed the discovery. In all, Segrè and Chamberlain and their group counted a total of 60 antiprotons, produced during a run that lasted approximately seven hours.

The antiproton discovery was greeted with great celebration in the scientific community and would win Chamberlain and Segrè the 1959 Nobel Prize in physics. In the popular press, however, it received mixed reviews.

Whereas the New York Times enthusiastically proclaimed "New Atom Particle Found; Termed a Negative Proton," the particle's hometown newspaper, the Berkeley Gazette, somberly announced "Grim New Find at UC." Apparently, the Gazette reporter had been told that should any of these new antiprotons come in contact with a person, that person would blow up.

Today, on their golden anniversary 50 years later, antiprotons have become a staple of high-energy physics experiments, with trillions routinely produced at CERN and Fermilab  and causing no known human fatalities.